This invention relates to a solid acid catalyst consisting essentially of acid-treated hydrated niobium oxide (or acid-treated niobic acid) a method for preparing the catalyst and the use of the catalyst in the hydration of olefins, such as ethylene.
Several procedures using sulphuric acid or aluminum chloride as catalysts are known in the chemical industry. There are many problems associated with the use of these catalysts. Among the problems are separation and recovery of the catalysts, treatment of water from the catalytic processes, corrosion of equipment and installations, formation of by-products, coloration of products, and so forth. In the case of aluminum chloride, there is also the problem of consumption of large quantities of the catalyst. It has been proposed to substitute mineral acids, such as sulphuric acid, phosphoric acid and the like, for solid acid catalysts.
There is practically no catalyst consisting of a solid acid suitable for use in the presence of water, except for certain ion exchange resins. These resins, however, have not been found to be entirely satisfactory because of their price, tendency to disaggregate and their limited range of application.
More recently, fluoride resin products, such as Nafion Type H, have been proposed. While these resins have a higher acidity than ion exchange resins, the acidity is still not sufficient and the resins are characterized by an extremely high price.
Hydrated niobium oxide (niobic acid) has been proposed as a solid acid catalyst. The niobium oxide catalyst can be prepared from hydrated niobium oxide (niobic acid) by washing the oxide with water followed by thermal treatment at low temperature. The hydrated niobium oxide can also be treated with sulphuric acid or hydrofluoric acid followed by a thermal treatment at low temperature, thus obtaining a niobium oxide catalyst which is a solid acid.
Even the niobium oxide catalysts have not proved entirely satisfactory. Crystallization occurs in these catalysts at high temperatures, such as a temperature at 400.degree. C., and mainly over a temperature of 500.degree. C. Crystallization in the catalyst results in deterioration of catalytic activity, thus prohibiting the use of these catalysts in reactions carried out at such high temperatures.
Niobium oxide catalysts that have lost some of their catalytic activity can be reactivated by heating the catalyst at an elevated temperature. If reactivation must take place at a temperature exceeding 400.degree. C., it is apparent that such a temperature could hardly be attained in practical terms to reactivate the catalyst, and this fact makes the utilization of hydrated niobium oxide catalysts difficult on an industrial scale.
Accordingly, there exists a need in the art for a solid acid catalyst that does not substantially crystallize at high temperature, does not exhibit a decrease of catalytic activity after exposure to high temperature, has improved surface acidity and is not poisoned by water.